docs: def guarded(self, func, *args, **kwargs):
          """ Call a function, return None on errors.
          """
          try:
              return func(*args, **kwargs)
          except (EnvironmentError, error.LoggableError, xmlrpc.ERRORS) as g_exc:
              if func.__name__ not in self.ERRORS_LOGGED:
                  self.LOG.warn("While calling '%s': %s" % (func.__name__, g_exc))
                  self.ERRORS_LOGGED.add(func.__name__)
              return None